In the optical communications space, various techniques are used to synthesize an optical communications signal for transmission. A popular technique utilizes a laser 2 coupled to an external optical modulator 4, as shown in FIG. 1a. The laser 2 generates a narrow-band continuous wave (CW) optical carrier signal 6 having a desired wavelength. The optical modulator 4 operates to modulate the amplitude and/or phase the carrier signal 6 to generate the optical communications signal 8 based on a drive signal 10 that encodes data to be transmitted. Typically, the drive signal 10 is generated by a driver circuit 12 based on an input data signal x(t).
In the arrangement illustrated in the FIGS. 1a-1b, the optical modulator 4 is provided by a well known Mach-Zehnder (MZ) interferometer. Other types of modulators may be used, depending on the desired type of modulation. For example, an electro-absorptive modulator (EAM) or a variable optical attenuator (VOA) may be used for amplitude modulation; whereas phase shifters are well known for implementing phase modulation. In each case, the driver circuit 12 generates the drive signal 10 by scaling the input data signal x(t) to satisfy the voltage and current requirements of the modulator 4. The driver circuit 12 may also generate one or more bias signals (not shown) for controlling a bias point of the modulator 4 in a manner well known in the art.
In the embodiment of FIG. 1a, the MZ modulator is controlled by a single drive signal 10 V(t), which, in this case, would typically be a scaled version of the input data signal x(t). In FIG. 1b, a “dual-branch” MZ modulator 4 is controlled by a pair of differential drive signals +V(t) and −V(t), which are substantially equal and opposite. The use of a differential pair of drive signals has the well known advantage that a desired amplitude modulation of the communications signal 8 can be obtained with drive signal voltage levels that are approximately half of that required for the single-drive embodiment of FIG. 1a. Additionally, it is possible to minimize undesirable signal chirp by adjusting the scaling factors applied to each of the differential drive signals +V(t) and −V(t)
In either of the above embodiments, the MZ modulator displays a sinusoidal response to the applied drive voltage (or voltage difference in the case of differential drive signals) as shown in FIG. 1c. Thus, a DC bias signal (not shown) is used to drive the modulator 4 to a desired bias point 14 of the response curve 16. The drive signal(s) 10 can then drive excursions of the modulator response, corresponding to maximum and minimum transmittance, respectively. This operation yields an amplitude-modulated optical communications signal 8 which carries the original data signal x(t), and is typically employed to implement On-Off-Keying (OOK) transmission protocols.
As is well known in the art, a directly analogous approach can be employed using a phase shifter as the optical modulator 4. In this case, the drive signal(s) 10 drive excursions of the modulator phase response, which yields a phase-modulated communications signal 8. This technique is typically used to implement Phase Shift Keying (PSK) or, more commonly Differential Phase Shift Keying (DPSK) transmission protocols.
A limitation of the optical signal synthesizers illustrated in FIGS. 1a–1c, is that they are designed to modulate only a single dimension (i.e. amplitude or phase) of the CW optical carrier signal 6 generated by the narrow-band laser 2. However, in some instances it is desirable to modulate two or more dimensions of the CW carrier 6. For example, Applicant's co-pending U.S. patent application Ser. Nos. 10/262,944, filed Oct. 3, 2002; Ser. No. 10/307,466 filed Dec. 2, 2002; and Ser. No. 10/405,236 filed Apr. 3, 2003; and International Patent Application No. PCT/CA03/01044 filed Jul. 11, 2003 describe techniques for compensating impairments in an optical link by predistoring an input signal, in the electrical domain, and then using the thus predistorted signal to drive the optical modulator. As described in those applications, successful implementation of this technique, particularly for the case of polarisation dependent and non-linear impairments, requires the use of an optical modulator capable of modulating both the amplitude and phase of the CW carrier 6. Stated more generally, it is desirable to modulate the E-field of the CW carrier, within the complex plane.
Various methods of accomplishing this result are described in Applicant's co-pending U.S. patent application Ser. Nos. 10/262,944, filed Oct. 3, 2002; Ser. No. 10/307,466 filed Dec. 2, 2002; and Ser. No. 10/405,236 filed Apr. 3, 2003; and International Patent Application No. PCT/CA03/01044 filed Jul. 11, 2003. As shown in FIGS. 2a and 2b, most of these techniques utilize multiple one-dimensional modulators in combination. Thus, for example, FIG. 2a shows a complex Mach-Zehnder modulator 18 composed of two 1-D MZ modulators 4a, 4b connected in parallel. A complex driver circuit 20 generates the drive signal(s) 10 in the form of In-phase and Quadrature signal components VI(t), VQ(t), each of which may be represented by a single signal or a differential signal pair. The drive signals 10 are used to drive a respective one of the branch MZ modulators 4a, 4b. This technique enables arbitrary E-field modulation of the CW carrier 6 throughout the complex I-Q plane.
In FIG. 2b, a conventional 1-D MZ modulator 4 is cascaded with a phase shifter 22. In this case, the complex driver circuit 20 generates the drive signals 10 in the form of amplitude and phase signal components VS(t) and Vφ(t) (each of which may be represented by a single signal or a differential signal pair), which are respectively used to drive the MZ modulator 4 and the phase shifter 22. This technique enables arbitrary E-field modulation of the CW carrier throughout the complex polar-coordinate (Amplitude-Phase) plane.
In FIG. 2c, the frequency dependence of conventional lasers on the drive current is used in conjunction with a conventional 1-D MZ modulator 4. In this case, the complex driver circuit 20 generates the drive signal(s) 10 in the form of amplitude and frequency signal components VS(t) and Vf(t). The amplitude component VS(t) (which may be represented by a single signal or a differential signal pair) drives the MZ modulator 4 to modulate the amplitude of the CW carrier signal 6 in a conventional manner. The frequency component Vf(t) provides the laser drive current, and is varied to induce desired excursions of the laser frequency. This technique enables E-field modulation of the CW carrier within the complex polar-coordinate (Amplitude-Phase) plane, limited primarily by the frequency response of the laser 2.
All of these prior solutions are advantageous in that they enable E-field modulation of the CW carrier 6. However, the solutions of FIGS. 2a and 2b are expensive, because multiple devices are required. The solution of FIG. 2c requires only a single amplitude modulator 4, but suffers a disadvantage that the laser is current-controlled. In some cases, obtaining the desired frequency modulation of the laser 2 may require very large and/or very rapid changes in the drive signal voltage. This increases the cost of the driver circuit 20, and may result in the production of unwanted noise.
Accordingly, methods and apparatus for cost-effectively modulating the E-field of an optical carrier signal remains highly desirable.